Albert Barnes Commentary Deuteronomy 18:21

"And if thou say in thy heart, How shall we know the word which Jehovah hath not spoken?" — Deuteronomy 18:21 (ASV)

And if you say in your heart, How... - The passage evidently assumes an occasion for consulting the prophet, similar to what was customary among pagans, e.g., before an impending battle or other such crisis (compare 1 Kings 22:11), in which his veracity would soon be put to the test. Failure of a prediction is set forth as a sure sign of its being "presumptuous."

But from Deuteronomy 13:2 and following, we see that the fulfillment of a prediction would not decisively accredit the one who uttered it; for the prophet or dreamer of dreams who endeavored on the strength of miracles to seduce people to idolatry was to be rejected and punished. Nothing, therefore, contrary to the revealed truth of God was to be accepted under any circumstances.